Meiosis and Sexual Reproduction

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Meiosis and Sexual Reproduction
Chapter 7

7.1 Meiosis In some organisms haploid gametes join to form a diploid zygote If gametes weren’t haploid imagine how the number of chromosomes would increase of time Meiosis is the form of cell division that makes diploid cells haploid when forming gametes There are 2 divisions in Meiosis – Meiosis I and Meiosis II Before Meiosis begins DNA is replicated so cell has homologous chromosomes

Prophase I Chromosomes condense Nuclear envelope breaks down
Homologous chromosomes pair up Crossing over occurs Chromatids exchange

Metaphase I Homologous pairs move to equator by spindle

Anaphase I Homologous pairs separate and pulled towards poles
Chromatids do not separate; still connected by centromeres

Telophase I Individual chromosomes gather at poles
Most organisms now have cytokinesis 2 new cells formed Chromosomes do not replicate again

Prophase II New spindle forms

Metaphase II Chromosomes line up along equator and are still attached at centromeres

Anaphase II Centromeres divide
Chromatids separate; Now called chromosomes

Telophase II Nuclear envelope forms around chromosome sets

Meiosis in Action

Meiosis and Genetic Variation
Three key ways meiosis creates new genetic variations Independent Assortment Crossing Over Random Fertilization

Independent Assortment
Most organisms have more than 1 chromosome In humans the gamete gets 1 chromosome from the 23 pairs Which of the pair you get is chance This random distribution is called Independent Assortment Mathematically 223 = over 8.3 million combinations

Crossing Over Portion of 1 homologous chromosome is broken off and exchanged with the corresponding portion of its homologous partner

Random Fertilization What egg and sperm meet is also completely random
Mathematically that means that the possible combinations of the sperm (about 8.3 million) can be multiplied by the combinations for the egg (about 8.3 million) giving 70 trillion possible combinations

Meiosis generates variation in a species very quickly
Pace of evolution is sped up due to variation A + B = C, a new individual not identical to its parents

Gametogenesis – Formation of Gametes
Spermatogenesis – sperm production occurs in testes Diploid cell increases in size and becomes immature germ cell Meiosis I = 2 immature sperm cells Meiosis II = 4 haploid cells Develop tails and become sperm

Oogenesis – egg production occurs in ovaries
Diploid cell increases and becomes germ cell Meiosis I = after cytokinesis cytoplasm is not equally divided Smaller one called polar body Meiosis II = larger cell divides again, again cytoplasm not equally divided Forms another polar body Polar bodies will die Ovum (pl. = ova) is larger cell More cytoplasm means more stored energy

7.2 Sexual Reproduction Asexual Reproduction
Single parent passes on copies to offspring called “clone” Identical to parent Prokaryotes use binary fission

Many eukaryotes use asexual reproduction
Fission – parent into 2 equal sized offspring Fragmentation Body into several pieces Missing parts grow back Budding – new split off from old; may stay attached

Genetic Diversity Asexual reproduction is simplest method
In stable environment can produce many offspring in short time’ Don’t need to waste energy making gametes or find a partner DNA varies very little In changing environment may not be able to adapt

Sexual Reproduction 2 parents form reproductive cells with half the number of chromosomes Diploid Mother makes haploid gamete Diploid Father makes haploid gamete Haploid gametes make diploid offspring different from parents

Evolution of Sexual Reproduction
May have come as a way for protists to repair DNA Only diploid cells can repair Protists form diploid cells under stress Many enzymes that repair DNA are involved in meiosis

Sexual Life Cycle in Eukaryotes
Life Cycle – span from 1 generation to next All sexually reproductive organisms have a diploid-haploid-diploid lifecycle Only 3 Types of Sexual Life Cycles Only difference is which phase becomes multicellular

Haploid – simplest and most common
Zygote is the only diploid cell and it undergoes meiosis immediately Haploid cells in multicellular organism produce gametes through mitosis Fusion – gametes fuse to form zygote and meiosis follows Meiosis allows for damage repair 2 homologous chromosomes are lined up, special repair enzymes remove damaged sections of DNA and fill in gaps Protists, Fungi, Algae

Haploid Life Cycle

Diploid Adults are diploid, inherit from 2 parents
Diploid cell then meiosis then haploid gamete Gametes join during fertilization forming diploid zygote Zygote divides by mitosis Gametes are the only haploid cells

Diploid Life Cycle

Parthenogenesis New individual forms from unfertilized egg
Hypothesized that mothers chromosomes are copied instead of getting fathers chromosomes Produces clones Honeybees provide drones this way (haploid) Whiptail lizards are all females Doesn’t seem possible in mammals Only mammal clones are twins from split egg

Alternation of Generations
Regularly alternates between haploid and diploid Plants – diploid phase called sporophyte produces spores Spore forming cells produce haploid spore that can form adult without another cell Gametophyte is haploid phase Produces gametes by mitosis Gametes fuse and produce diploid phase Diploid – Haploid – Diploid – Haploid – etc… Moss – haploid spores develop at top of sporophyte stalk